I had two more small head amp projects in the hopper, and I finally got around to complete one.
There is a fellow in Paris, France that saw the TI just designed a new very small op amp specifically for headphone amps and had
to try it out!
TI has design circuits, test circuits etc.."Chris" designed a circuit and layed out a PCB and I wanted to hear this new headphone amp chip the OPA 1622.
Heres the link to the TI datasheet: http://www.ti.com/product/OPA1622/datasheet
The dimensions of this 10 pin chip: OPA1622 VSON (10) 3.00 mm × 3.00 mm
This presents a problem for us old school solder DIY'ers!
It made me buy a hot air gun station, learn how to use solder paste and do micro soldering !!!
It was at first intimidating, but after some practice and sitting on the project for a few weeks I started!!
The OPA1622 is U4 all hot air gunned down....notice the 2mm x 2mm IC on yet soldered down!
All 5 of the IC's a vregulator 7805 are installed.
Here is the rest of the goodies all nice and neat ready for testing!
What it looks like packaged up less a volume knob:
Here are Chris's design comments:
Like some of you I followed with great interest the thread 'New Audio Op Amp - OPA1622' by TI’s Johnnc124. I wasn’t initially specifically interested by headamps but as the OPA1622 was simple to implement and was supposed to provide good results I decided to setup a project with it. More than building an headamp this project was aimed to confirm I could achieve designing a PCB with a CAD software and also soldering SMD devices, including ones in DFN or QFN package.
I’have finally been through all steps, I’m happy with the result (OPA1622 sounds very good !) and I’m now confident to move to projects that will require complicated PCB and good soldering capabilities.
Now, I have to finish assembly and testing of a Salas DGC3 in order to do some comparisons!
The main requirements for AmpCasq where :
It should stand in a small aluminum box, in order to sit on desk
It should be powered by an AC adapter I had, which provided 17VDC
It should have a volume control
It should have RCA and jack 3.5 inputs
It should have jack 3.5 & 6.35 outputs
It should allow me to experiment crossfeed
Final price should stay under control
About the schematics:
Ampcasq was designed in September 2016, at this time LM27762 was announced but not available. So, I did stay with LM27761 for -5V and LP38798 for +5V; LP5907 was a bit short on current. The 7805 lifted by R17/R12 was added to limit LM27761 input voltage to 5.5V, its max value. 7805 allow Ampcasq to be powered by any DC source from 10V to 35V, 300mA max.
D1 to protect against bad connection.
D2, R23, R10, R11, C24 to manage the Enable pin of the OPA. Objective was to avoid noise in the can at power up/down. This partly works: mute the output when enable Low (<~0.8V) but can’t avoid noise in can when power supply capacitors are discharching and OPA1622's supply getting out of tolerance. On power off with SW1, OPA1622 is muted as C24 is quickly discharged through R23, D2 and SW1. D2 is a Schottky, not for speed but for low drop (Vf close to 0.3V).
It is based on an Xen-Audio (EUVL) paper: Xen Audio cross feed.
There’re three positions for the switch: Low Xfeed – No Xfeed – High Xfeed. Low Xfeed is Ok, High is exaggerate: there’s too much bass boost. After some listening sessions I finally do not use the Xfeed.
Gain + out
Not much to say.
I will do much listening and compare to my other amps and projects...so far it worked the first time powered up!!
This is always a good thing!
All the best